This invention relates to automated proofing for color image processing.
Commercial-scale production of color documents includes a proofing step in which a pre-production “proof” copy is printed. The proofing process verifies that the proof copy corresponds to the expected print quality prior to performing the production “press” print, which may include several batches or copies.
Proofs originated in commercial printing when printers used lithographic (for publication), gravure (for package printing) and flexographic (long-run) processes. The proof provides an economical preview print for customer review before committing effort and materials in creating printing plates, inking the press and adjusting ink and water balance. Current technology, such as a low-cost color printers, enables designers and photographers to make proofs.
Proofs include two types, depending on their use: contract proofs and design proofs, also known as “comps” or preproofs. Contract proofs show appearance of an accurately reproduced image when printed, for the customer to approve before press production. Design proofs show the early stage of a print job, generally for layout, position and general appearance, with less emphasis placed on accurate color, except for final comps.
The characteristics of a device may be described by a color profile. Color devices typically operate within a device-dependent color space that represents a mapping coordinate system. The color profile may describe a relationship between the actual color produced in a native color space and an abstract (device-independent) color space. A device-specific color profile may correspond to a generalized class of devices, a generic factory profile from the manufacturer, or a custom profile based on calibration for a particular device.
These profiles may define the boundaries of the gamut, which may represent the total color volume capable of being rendered by the device to which the color profile corresponds. The gamut of one device may differ considerably from the gamut of another device. A proof printer, for example, may employ a larger gamut than a typical press or production printer.
An imaging workflow may represent a path from an input (image capture or receiver) device from which an image may originate digitally to an output (printer or display) device to which the image may be respectively printed or displayed. The image may be encoded in digital form for rendering. The input device may be represented by an image capture device, such as a scanner or a digital camera, or by a file receiver device that maintains a preexisting image. The output device may be represented by a printer (as an output device) or by a screen monitor (as a display device).
Color management may include obtaining color profiles that describe all devices employed between the input and output devices. Combining these profiles into a workflow may include transformation from one color space used for a first color profile to another color space used for a second color profile through a profile connection space (PCS).
PCS represents a standard or reference color space that may provide an unambiguous connection between input and output color spaces. PCS may be device-independent, also called abstract, such as CIE XYZ (1931) developed by the internationally recognized Commission Internationale de 1'Eclairage (CIE), to produce an International Color Consortium (ICC®) profile. The workflow may employ PCS to link across device-dependent color spaces.
Transformation from one device-dependent color space to another device-dependent color space may cause loss of color information from non-coextensive gamuts or ambiguities due to different interpretations of one or more colors between devices. A rendering intent may be selected to specify which parameters to compromise when mapping from input to output color spaces.
A color management module (CMM), also called a color engine, may be used to assign a profile to an image or a job, and also to perform an actual transformation from the input color space to the output color space. The CMM may be incorporated in the device or elsewhere in the workflow. Color management describes the color of pixels and converts their values to maintain consistent color across various devices.
An image capture device may convert a source image into digital image data and assign an input profile describing information for converting from the gamut of the image capture device to PCS. An image output or destination device, frequently including a digital front end (DFE) for processing received image data, may render digital images to hardcopy or softcopy media. The DFE may include a raster image processor (RIP), such as for a digital color press. The RIP may convert postscript and portable document format (PDF) files into raster images that may be submitted to the color press or to a digital plate maker for imaging.
Some image devices (for input and display), such as digital cameras and liquid crystal diode flat-screen monitors, may use an additive (e.g., primary-based) color space, such as red-green-blue (RGB) coordinates. Printers typically may employ a subtractive (absorbent) color space, such as cyan-magenta-yellow-black (CMYK) coordinates.
Because the RGB and CMYK color spaces may correspond to a specific device or hardware model, the ranges of these respective spaces may not cover the same gamut or region, and thereby may preclude complete mapping correspondence between the RGB and CMYK spaces. Conversion of color spaces from the typically larger RGB gamut to the typically smaller CMYK gamut may yield losses in color mapping information.
PCS may provide a common interface between input, display and output devices to allow their respective profile transforms to be decoupled from each other. Converting an image source to a destination device color space for printing may entail mapping from the capture RGB coordinates to PCS and then mapping from PCS to CMYK printing coordinates. The ICC profile may provide information on converting from a device-dependent color space to PCS.
Under inline proofing, the same color printer device that prints the proof also prints the final print job. Establishing a match between the proof and the final print is performed based on the print-to-print variation of the color printer.
Under offline printing, a pre-production proofing device prints the contract proof, and subsequently a press (for commercial production printing) prints the final print run. Establishing a close match between the contract proof and the final print may involve a procedure for obtaining similar colors in prints produced from separate devices. Each printer may operate over a gamut, typically mapped in coordinates of a CMYK color space.
One technique for offline printing comparisons uses a lowest common denominator in a CMYK color space, such as Specifications Web Offset Publications (SWOP®) under U.S. Web Coated v. 2. However, SWOP employs a restricted gamut considered inadequate for some professional applications. For an image that includes colors not encompassed by a SWOP-profiled printer, the color boundaries may be truncated by the printer.
Another technique uses an RGB workflow with ICC profiles. The offline printing comparison using the ICC profile-based RGB workflow enables the proofing device to match a greater variety of the high-production press, because larger color gamuts are available than in SWOP color space. This technique may benefit digital production presses, which generally use higher-quality inks than offset presses, and hence may have a larger gamut than offset presses.
Prior to ICC color management, proprietary software were used to convert from an input device-dependent color space, such as RGB, to an output device-dependent color space, such as CMYK. The ICC color management workflow uses open-architecture formats, and may employ a device-independent color reference space to serve as PCS.
The workflow may provide a process for transforming color spaces for received and transferred image data to a device-independent PCS for the ICC profile. An output device receiving the image data may also receive, through the workflow, the ICC profile to correctly process the image data.
The typical RGB workflow uses several profiles for translating between the proofing and production devices. An input profile may describe the source or image-receiving device (e.g., scanner, camera) used to digitize the original image. An output profile may describe either a proofing device for producing a proof of the image or a production press for producing printed copies of the image. A simulation profile describes an emulation device that simulates the production device.
Conventionally, to establish a match between the offline proofing device and the production device, a user must embed the input profile (from the image digitizer) used to digitize the images to be processed, and embed the output profile (from the printing device to be used). For example, the input profile may represent the digitized image data together with the source profile of the image capture device. Optionally, the user may embed a simulation profile that describes the production device that the proof printer will emulate.
To enable an offline proofing device to match the press, the conventional technique requires input profiles for all images to have embedded therein: the input profile, a proofing device profile from the proofing device as the output profile, and a press profile from the press as the simulation profile. A print job may then be executed on the proofing device. Following customer approval of the proof print, the user must reprint the print job from the input profile, delete the simulation profile and replace the output profile with the press profile that corresponds the press, rather than the proofing device.